Thorium and uranium phosphate syntheses and lixiviation tests for their use as hosts for radwastes

Thorium and uranium phosphates were synthesized via soft chemistry and then calcined at high temperature, respectively up to 850 and 1,170 C. Th3(PO4)4 was not found. Instead, the authors found they had made a new mixed valence compound: U(UO2)(PO4)2, for which crystal structure, absorption spectrum in ultraviolet and visible, and X-ray photoelectron spectroscopy were done. All of these experiments confirmed the presence of uranium in both tetravalent and hexavalent states. Lixiviation of thorium phosphate labeled with radioactive isotopes: 227Th and 223Ra allowed them to determine the upper limit of the solubility for this compound (about 7.6 x 10-11 M) and, the percentage of 223Ra released in deionized water (about 5 x 10-3%). Similar lixiviation tests on U(UO2)(PO4)2 labeled with 230U (radiochemically produced) gave a uranium solubility value 10 times greater than uranium measured by laser spectrofluorometry. This discrepancy is attributed to the radiation chemistry inherent in the use of radioactive isotope at high specific activity. The lixiviated uranium fraction from U(UO2)(PO4)2 is seven to eight orders of magnitude higher than for thorium. Unexpected behavior was observed for uranium solubility which increases by two orders of magnitude after 2,000 hours of lixiviation. This phenomenon is not yet understood